I want to see Kara in the next film, and I believe she could serve the plot well. If Lex Luthor is in the next film, I would love to see him as a shrewd, power-hungry businessman who mounts a xenophobic presidential campain against Kryptonians and aliens in general.

Superman integrates well into human society because he was a life birth who was raised by a middle American family. Kara, however, was probably genetically engineered to be a scout. In theory, she would be unable to integrate into human society, and I could see Lex exploiting this in order to portray her (and by extension Superman since they both would wear the crest of the House of El) as incompatible with humans.